Process for the production of dialkyl sulfoxides



y 3, 1960 F. HUBENETT EI'AL 2,935,532

PROCESS FOR THE PRODUCTION OF nmum. SULFOXIDES Filed Nov. 12, 1957 INVENTORS. FRITZ HUBENETT KARL-HEINZ KEIM BY MMW 423: A T TORNE Y5 PROCESS FOR THE PRODUCTION or DIALKYL surroxmns Fritz Hiibenett and Karl Heinz Keim, Wesseling, Germany, assignors to Union Rheinische Braunkohlen Kraftstolf Aktiengesellschaft, Wesseling, Germany, a corporation of Germany Application November 12, 1957, Serial No. 695,966 Claims priority, application Germany November 15, 1956 '4 Claims. (Cl. 260-607) This invention relates to an improved and economical process for the production of dialkyl sulfoxides.

It is an object of the present invention to provide an improved and economical process of producing dialkyl sulfoxide by oxidizing dialkyl sulfide.

Another object ofthe present invention is to provide a process of producing dialkyl sulfoxide by oxidizing dialkyl sulfide without any loss of sulfides occurring in the oxidation process and while reducing loss of nitric oxides to a minimum.

A still further object of the present invention is to provide a process of producing dialkyl sulfoxide which is free from nitrogen.

Further objects of the invention will become apparent as the following description proceeds.

It is known to produce dialkyl sulfoxides by oxidizing dialkyl sulfides with oxygen or gases containing oxygen in the presence of nitric oxide either in the vapor or liquid phase.

.The aforesaid prior art processes suffer from serious disadvantages. In the first place, they do not prevent loss of nitric oxide entrained with the Waste gases and, especially, of the liquid reaction product. The dialkyl sulfoxides obtained by the hitherto known processes have a nitrogen content of more than 3%. Reduction of the nitrogen content by physical means involves diflicultiesl The nitrogen content can be reduced to 12% only, for instance,'by heating the reaction product to 100 C. and passing air therethrough. A product of high purity, which contains nitrogen in a proportion of, for instance,

0.1%, can be obtained therefrom only by distillation in vacuo with removal of a sniall proportion of a first running and a last running. This reduces the yield of the pure product to about 90%. Another considerable disadvantage of the previously used'processes is thatit is almost impossible to avoid loss of the readily volatile dialkyl sulfides-for instance dimethyl sulfide has a vapor pressure of 100 mm/Hg at --12 C. I This loss can be kept Within tolerable limits only if, instead of air or gases containing oxygen, oxygen is used for oxidation. However, when the prior art processes for the production of dialkyl sulfoxides from dialkyl sulfides are carried out continuously, then, regardless of Whether oxygen, air or a gas containing oxygen is used for oxidation, it is quite impossible to avoid loss of both sulfides and nitric oxides.

The present invention is based on the high solubility of nitrogen dioxide and the low solubility of nitric oxide in a number of solvents, especially in dialkyl sulfoxides. In accordance with the invention, dialkyl sulfoxides of very high purity are produced from dialkyl sulfides by oxidizing the sulfide with a solution of nitrogen dioxide, preferably in the dialkyl sulfoxide obtained as reaction product. The term nitrogen dioxide as used herein is intended to denote the equilibrium mixture of nitrogen dioxide and di-nitrogen tetroxide. 7

It is possible according to the invention to use other solvents, especially chlorinated hydrocarbons, but the use 2,935,532 Patented May 3, 1960 the complete reduction of the nitrogen dioxide to nitrogen monoxide which escapes in the form of gas.

The nitrogen monoxide may be converted in a second reactor into nitrogen dioxide with oxygen or a gas containing oxygen and separated from the waste gases in a tower by washing with a solvent, preferably the sulfoxide obtained in the reaction. The nitrogen dioxide solution thus obtained is returned to the first reactor and reused for oxidizing more of the sulfide. After the excess sulfide has been separated out, which can easily be efiected by heating and, if desired, expelling with a gas stream, a

sulfoxide of high purity is obtained. Part of the sulfoxide can then be returned to the scrubbing tower, Where it is used to wash out the nitrogen dioxide.

bing tower.

The invention will now be further described with refer-- ence to the single figure of the accompanying diagram matic drawing, which is a fiow sheet of one embodiment of the process of the invention.

A dialkyl sulfide is fed through a line a to a reactor A,. which'is filled with the sulfide, and is contacted in the' reactor A with a solution of nitrogen dioxide in dialkylt sulfoxide which flows into the reactor A through a pipe b. When nitrogen dioxide is present in relatively high con-- centration, oxidation of the sulfide instantaneously occurs with violent evolution of nitric oxide and is substantially" complete when the reaction mixture consisting of sulfide and sulfoxide flows via a pipe d into an extraction vessel? B the upper part of which may be constructed as a column. Care is taken that the reaction solution is intimately mixed in the reactor A; evolution of nitric oxide is utilized to eflfect mixing according to the principle of an air-lift pump. Alternative methods of mixing the reactants can be used, for instance pumping or stirring. The nitric oxide evolved inreactor A passes through a line c into the heated lower part of the extraction vessel B. Assisted by the nitric oxide gas stream which flows upwardly in the extraction vessel B, the mixture of sulfide and sulfoxide which enters the vessel B via the pipe d is substantially separated into its components, and the sulfide vapors are led through a pipe 2 to a cooler C Where they are condensed. The sulfide is then returned to the reactor A via pipe 1 and a.

The sulfoxide, which is substantially free from sulfide, passes through a pipe g into a heated separation vessel D through the bottom of which a stream of air or oxygen is introduced via a pipe h. In the separation vesselD the sulfoxide is free from the last traces of sulfide, and

I the resulting sulfoxide is removed from the apparatus via a pipe i. Some of the sulfoxide is led away through a pipe j, the rest is fed via a pipe k to a scrubbing tower F which is filled with packing material. In the scrubbing tower F, the sulfoxide isagain saturated with nitrogen dioxide and the resulting solution is returned to the reacpipe in entrains a proportion of sulfide which depends upon the temperature prevailing in the cooler; the nitric oxide combines with the air or oxygen which is introduced through the pipe 1 into the top of the oxidation vessel G. Nitrogen dioxide is instantaneously formed, and converts the entrained sulfide vapors into sulfoxide, which is' con- The use of' solvents other than the dialkyl sulfoxide requires the in-- terposition of an apparatus for separating oif the sulfoxide: or the solvent, which can then be returned to the scrubdensed and flows into the reactor A, together with dissolved nitrogen dioxide, via a pipe 11, the lower part of a secondary reactor H, which is constructed as a separ -a-tor,--'andpipes'o and b. In the oxidation vessel G only part of the nitric oxide is reacted to form nitrogen dioxide. Conversion is completed in the secondary reactor H, from which the nitrogen dioxide flows to the scrubbing tower F via a pipe p. In the scrubbing tower F, the nitrogen dioxide is dissolved by sulfoxide running down, and the solution is recirculated via pipe b While the waste gases are led off via a pipe q.

Since the reaction 2N0+O 2NO has a negative temperature coefficient, it is advantageous to cool theparts G and H; G is preferably merely cooled with water to prevent the'sulfoxide formed therein from crystallizing. Moreover, it is advisable to keep the cooler C at a temperature as low as possible in order to minimize the undesired oxidation of sulfide in the vessel G in the gas phase by reducing the sulfide vapor pressure. Oxidation in the gas phase leads to the reformation of nitric oxide, which slows down the production of dioxide and necessitates the use of a secondary reactor H of larger dimensions. Independently of the reaction of nitric oxide to form nitrogen dioxide at higher speed, which can be accomplished by cooling the oxidation vessel G and secondary reactor H, it is also possible to effect complete conversion by the use of a secondary reactor H of larger dimensions.

Nitrogen oxides can be fed in through a'pipe 1' when the plant is putinto operation. Any losses of nitrogen oxides which may occur due to careless operation can be compensated by making up' with fresh quantities of nitrogen oxides fed in through the pipe r. Instead of nitrogen oxides, fuming or concentrated nitric acid can be used, which since it forms nitrogen oxides by reaction with the dialkyl sulfide, the nitric acid is preferably introduced at the bottom of the tower F. However, the use of nitric acid requires the inter-position of a water separator in pipe 1, since dimethyl sulfide, for instance, forms an azeotrope with water. Besides, aqueous sulfoxide is formed in pipe j if nitric acid is used. The quantity of air entering via the pipe h is preferably adjusted so that the waste gas leaving via the pipe g still contains oxygen. The proportion of oxygen in the waste gas 'is preferably not less than 7% by volume. Of course, the danger of losses of nitrogen oxides occurring is reduced when oxygen is used instead of air, but, on the other hand, this may lead to the formation of sulfoxide containing traces of sulfide in the vessel B if the boiling in vessel B was insuificient. According to a preferred embodiment of the invention, a gas is used which is richer in oxygen than air. A gas having a high proportion of oxygen can for instance be obtained as waste gas in certain air liquefying plants or can readily be produced by adding oxygen to air.

In order to be able to control the reaction, it is preferable to provide the reactor A, or at least the lower section of the reactor A, with cooling andheatin'g means. Heating is especially necessary if nitric acid is used instead of nitrogen oxides. On the other hand, especially when the hereindescribed process is carried out in apparatus of large dimensions, it may be advantageous to coolthe reactor A in order to minimize side reactions and losses of the reactants or reaction product.

As may be seen from the following examples, which illustrate the process of the invention, this process is an economical process which substantially avoids any losses of sulfides, While reducing losses of nitrogen oxides to a minimum, and produces a nitrogen-free'crude product in very high yield without any after-treating steps being necessary, for instance distillation in vacuo.

Example 1 The hereindescribed process was carried out in a glass apparatus, as shown in the single figure of the accomdioxide solution is reacted with an excess of the dialkyl panying drawing, most of the parts having ground joints; for instance a 45 cm. long Dimroth cooler was used as the oxidation vessel G. The cooler E and the oxidation vessel G were cooled with water, the cooler C and the secondary reactor H with brine at a temperature of -10 .to l5 C. The sulfoxide leaving via the ipei was brought to room temperature in a water cooler and part of the sulfoxide was introduced in portions into a storage container mounted on the scrubbing tower F and continuously fed from the storage container into the tower. After the apparatus had been operated for an initial starting period of 24 hours, the yields and results which were obtained in the next 24 hours were as follows:

792 g. of dimethyl sulfide were used as starting material. 25 cc. of dimethyl sulfoxide were continuously introduced per hour into the scrubbing tower F. 40 l./h. of air were admitted through pipe [2. The waste gas, amounting to about 34 1./h., had an average oxygen content of 7.4% and contained only approximately 9 mg./l. of nitric oxide; this loss was compensated for by introducing 1 litre of nitric oxide every 4 hours through pipe h.

By using a secondary reactor H of an increased size,-

the losses of nitric oxide could be avoided practically completely. The waste gas did not contain any detectable quantities of sulfur-containing compounds. sample of gas withdrawn through pipe m contained approximately of nitric oxide, the remainder consisting of dimethyl sulfide and traces of nitrogen.

After deducting the sulfide recirculated, 980 g. of nitrogen-free dimethyl sulfoxide (-M.P. 17 C.), i.e. more than 98% of the theoretical, were obtained through pine 1'.

Example 2 The following process was carried out in the apparatus described in Example 1.

I Dimethyl sulfide was introduced in the quantity used in Example 1, but only 15 litres of air were introduced per hour through pipe h and. 5 litres of oxygen into pipe I before the cooler B. About 14 l./h. of waste gas containing 16% of O and onlyabout 0.7% of nitric oxide, calculated on the nitrogen dioxide recirculated per hour, were obtained. The loss of nitric oxide was replenished by feeding in 1 litre of nitric oxide every 12 hours. The waste gas-was free from dimethyl sulfide. The yield of pure sulfoxide was more than 98% of the theoretical.

We claim:

1. A process for the production of dialkyl sulfoxides of high purity, which comprises the liquid phase of oxidation of a stoichiometric excess of a dialkyl sulfide with a solution'of nitrogen dioxide in a dialkyl sulfoxide.

- 2. A process as claimed in claim 1, wherein the nitric oxide set free by the oxidation is separately reacted with oxygen to form nitrogen dioxide, this nitrogen dioxide is recovered from the last-mentioned reaction by dissolving it in some of the dialkyl sulfoxide formed by the oxidation, thus reforming the nitrogen dioxide solution required for oxidation, and the said nitrogen dioxide solution is used for additional oxidation.

3. A process as claimed in claim 1 wherein the nitrogen sulfide to give'substantially complete conversion of the dissolved nitrogen dioxide into nitric oxide, and, wherein excess dialkyl sulfide is separated from the liquid oxidation' products by blowing through the liquid products a stream of the nitric oxide formed by the oxidation, and any dialkyl sulfide left in the so treated oxidation products is separated off by heating and by treatment with gases containing free oxygen, and the resulting gases are subsequently used to oxidize said nitric oxide.

4. A process as claimed in claim 1 comprising the steps of heating the obtained dialkyl sulfoxide to separate therefrom the greater part of the unreacted dialkylsulfide and the nitric oxide set free by the oxidation, returning said greater part ofthe'dialkylsulfide to the liquid phase oxida- 5 7 tion, separating from said dialkyl sulfoxide the remainder and introducing both said solutions in the liquid phase of the dialkylsulfide by heating and passing free oxygen oxidation of the dialkyl sulfide. through said dialkyl sulfoxide, reacting said nitric oxide with the mixture of said remainder of the dialkylsulfide References Cited in the file of this patent and free oxygen to form a solution of nitrogen dioxide in 5 dialkyl sulfoxide, absorbing in dialkyl sulfoxide the ex- UNITED STATES PATENTS cess of nitrogen dioxide evolvedfrom said solution to 2,581,050 Smedslflnd June 1, 1952 form new solution of nitrogen dioxide in dialkyl sulfoxide, 2,702,824 Wetterholm 6t 955 

1. A PROCESS FOR THE PRODUCTION F DIALKYL SULFOXIDES OF HIGH PURITY, WHICH COMPRISES THE LIQUID PHASE OF OXIDATION OF A STOICHIOMETRIC EXCESS OF A DIALKYL SULFIDE WITH A SOLUTION OF NITROGEN DIOXIDE IN A DIALKYL SULFOXIDE. 